Metallic catalyst and aluminum oxide containing supports from acid leached alloys

ABSTRACT

AN IMPROVED METALLIC CATALYST OF HIGH SURFACE AREA A PREPARED FROM AN ALLOY COMPRISING ALUMINUM AND A SECOND METAL WHICH IS SOLUBLE IN OXIDIZING ACIDS. THIS ALLOY IS CONTACTED WITH AN OXIDIZING ACID FOR A TIME PERIOD AND AT A TEMPERATURE AND ACID CONCENTRATION SUFFICIENT TO DISSOLVE A PORTION OF THE SECOND METAL. THE ALLOY IS SUBSEQUENTLY CONTACTED WITH AN ALKALI METAL HYDROXIDE UNDER CONDITIONS SUFFICIENT TO CONVERT THE ALUMINUM AT THE SURFACE OF THE ALLOY TI O ALUMINUM HYDROXIDE. THE ALLOY THUS TREATED IS THEN HEATED IN THE PRESENCE OF OXYGEN FOR A TIME PERIOD AND AT A TEMPERATURE SUFFICIENT TO CONVERT THE ALUMINUM HYDROXIDE TO ALUMINUM OXIDE.

United States Patent 3,712,856 METALLIC CATALYST AND ALUMINUM OXIDE CONTAINING SUPPORTS FROM ACID LEACHED ALLOYS Erwin C. Betz, 524 Mill Valley Road, Palatine, Ill. 60067 No Drawing. Filed Dec. 2, 1970, Ser. No. 94,627 Int. Cl. C23b /02; B01j 11/08, 11/06 US. Cl. 204-29 50 Claims ABSTRACT OF THE DISCLOSURE An improved metallic catalyst of high surface area is prepared from an alloy comprising aluminum and a second metal which is soluble in oxidizing acids. This alloy is contacted with an oxidizing acid for a time period and at a temperature and acid concentration sufiicient to dissolve a portion of the second metal. The alloy is subsequently contacted with an alkali metal hydroxide under conditions sufiicient to convert the aluminum at the surface of the alloy to aluminum hydroxide. The alloy thus treated is then heated in the presence of oxygen for a time period and at a temperature suflicient to convert the aluminum hydroxide to aluminum oxide.

The present invention relates to a method for preparing improved metallic catalysts and catalyst supports having high surface area and to the catalysts and catalyst supports made thereby.

Generally, the catalysts of the present invention are prepared from an alloy which comprises aluminum and a second metal, the second metal being soluble in oxidizing acids. In carrying out the method, the alloy is first contacted with an oxidizing acid for a time period and at a temperature and acid concentration suflicient to dissolve a portion of the second metal. Subsequently, the alloy is contacted with an alkali metal hydroxide under conditions sufl'icient to convert the aluminum at the surface of the alloy to aluminum hydroxide. The alloy is then heated in the presence of oxygen for a time period and at a temperature sufficient to convert the aluminum hydroxide to aluminum oxide (alumina).

It is well known in the art that the efficiency of a catalyst may be increased by increasing the surface area. It is also well known that alumina, particularly in the gamma form, not only has catalytic activity by itself, but also serves as an excellent catalyst support, particularly for noble metals. In accordance with the present invention, a portion of the metal making up the alloy employed is dissolved in the oxidizing acid, while the aluminum is not, thus creating tiny valleys where the dissolved metal is removed, while leaviing peaks where the aluminum remains. The presence of these peaks and valleys creates a high surface area.

It is important that the acid employed for the acid treatment step be an oxidizing acid. The reason for this requirement is that the oxidizing nature of the acid produces an oxide film on the surface of the aluminum contained within the alloy. This oxide film has a very low solubility in the acid, and protects the aluminum from attack. The second metal, which on the other hand, is soluble in the oxidizing acid, is attacked and dissolved by the acid. While it is possible that oxidation as well as dissolution of this second metal occurs, the presence of the two phenomena makes no difference, and for purposes of the present invention, metals which are soluble in oxidizing acid includes metals that are oxidized by the oxidizing acid, and that have oxides that are soluble in oxidizing acids.

The preferred oxidizing acids for use in accordance ice with the present invention, is nitric acid. However, other acids, such as phosphoric acid and aqua regia, can also be employed.

A wide variety of aluminum-containing alloys may be employed with the present invention. As a general matter, such alloys should contain from about 4.5 to about 25% aluminum by weight, and preferably such alloys comprise about 4.5 to 15% aluminum because of the superior mechanical properties of alloys with lower aluminum content.

The identity of the second, oxidizing acid-soluble metal depends upon a number of factors, the most important of which are the physical and mechanical properties desired in the alloy. The most preferred second metal is iron, which may be present in amounts from about 6% to about by weight. Alloys containing only aluminum and iron are highly suitable. Other second metals that may be alloyed with the aluminum, and which are also soluble in the oxidizing acids, include nickel, molybdenum, titanium, vanadium and tungsten. Combinations of these metals, with or without iron, can also be employed. Still other metals may also be used provided that a suitable alloy may be prepared with aluminum, and also provided that these other metals are soluble in oxidizing acids. As with iron, the foregoing metals may be alloyed with the aluminum in total amounts ranging from about 6 to 85 by weight.

Further metals may also be incorporated into the alloys in addition to those previously mentioned. Again, such further metals are often advantageously added to improve the mechanical properties or the heat resistance of the alloy. Suitable additional oxidizing acid-soluble metals are chromium, cobalt, and manganese. Of the latter metals, chromium is particularly suitable, and chromium containing alloys have excellent characteristics in high-temperature applications. When chromium, cobalt, and manganeses are employed, they may be incorporated into the alloy in amounts up to about 32%. Above about 32%, the alloy sufiers a severe loss in mechanical strength. While there is: no required minimum, generally little effect on the alloy is seen when the chromium, cobalt, and/or manganese is below about 3% by weight.

When the alloy in accordance with the present invention contains more than two metals, there will be varying degrees of dissolution by the oxidizing acid, thus creating a very high surface area, having peaks and valleys of varying height and depth. A particularly suitable alloy for use in accordance with the present invention is one containing about 4.5 to 15% by weight aluminum, about 6% to 85 by weight iron, and anywhere from 0 to 32% chromium. An especially preferred alloy contains about 70% iron, about 5% aluminum, and about 20% chromium, the remainder of the alloy being made up of additional metals and non-metals in minor proportions, such as cobalt, silicon, and manganese.

The particular form of the metal is unimportant, except that it should be easy to handle. Generally, metal ribbon or thin metal sheets are preferred, although the method of the present invention may be employed to prepare catalysts in the form of screens, granules, or any other desired form.

As previously mentioned, the preparation of catalysts in accordance with the method of the present invention is commenced by contacting an alloy as described above with an oxidizing acid for a time period and at a temperature and acid concentration sufiicient to dissolve a portion of the oxidizing acid-soluble metal. The acid concentration may be varied over a wide range, generally any where from about 2.5% to by weight. Fuming acids may also be employed. It is preferable, however, to maintain the acid concentartion in the range of about 8% to about 25% by weight, as better control of the metal dissolution can be exerted.

The temperature at which the acid is contacted with the metal may also vary over a wide range, and is not critical. Of course, with less concentrated acids, high temperatures ar often required in order to obtain acceptable dissolution speeds. As a general matter, the temperature of the acid may vary anywhere from a point just above the freezing point of the acid to very high temperatures, up to the decomposition temperature of the acid, provided that pressure is employed to prevent the acid from boiling away. As a practical matter, how ever, the treatment temperature should be at least about 20 C., preferably in the range of about 20 to 100 C., and more preferably in the range of about 20 to 75 C.

As mentioned above, the time of treatment will also vary over a wide range, and will be dependent upon both the temperature and the acid concentration. The time temperature, and concentration factors should always be regulated so that only a portion of the oxidizing acidsoluble metal is dissolved. That is, if the treatment is too severe, the alloy can 'be greatly weakened mechanically, or even caused to disintegrate. On the other hand, if the conditions are insufiiciently severe, the surface area may not be increased as much as desired. Regarding time of treatment, as a practical matter, treatment times in the range of about one minute to 90 minutes may be employed, with times in the range of about 2 to 20 minutes being preferred. Of course, in addition to the reaction conditions and acid concentration, the composition of the alloy itself, including the proportion of aluminum and the identity of acid-soluble metals, must be taken into consideration. That is, the lower the percentage of acid-soluble metals, and the less the acid solubility, the more severe are the conditions that must be employed.

Subsequent to the acid treatment, the alloy is preferably rinsed, and is then ready for the conversion of the aluminum to aluminum hydroxide. As previously mentioned, the aluminum at this point, at least at the surface, will be in the form of aluminum oxide as a result of the action of the oxidizing acid. In order to further increase the surface area of the catalyst, this aluminum oxide is next converted to aluminum hydroxide, which forms a porous gel of high surface area. The hydroxide is then reconverted to the oxide, and preferably calcined to convert it to gramma alumina, a crystalline form of high catalytic activity.

In the conversion to aluminum hydroxide, the alloy is treated with an alkali metal hydroxide. The conditions must be suflicient to convert the aluminum at the surface of the alloy (which is mainly in the form of aluminum oxide) to aluminum hydroxide. Although molten alkali metal hydroxide may be employed, the hydroxide is preferably in the form of an aqueous solution ranging in concentration from about 5% by weight to a saturated solution. In the preferred embodiment, the concentration of the alkali metal hydroxide is about 50 to 87% by weight. The preferred alkali metal hydroxide is sodium hydroxide.

The time and temperature of treatment may also be varied over a wide range. Neither of these factors is critical, and they are interrelated to each other as well as to the concentration of the solution. However, in most situations it is preferred to maintain the temperature in the range between the freezing point and the boiling point of the solution, as operation in a pressurized system is inconvenient and unnecessary. Preferably the temperature is maintained in the range of about 20 to 100 C. The time of treatment also is not critical, but as a practical matter should be maintained in the range of about 2 to 90 minutes, and preferably about 20 to 40 minutes. The alkali metal hydroxide treatment is most preferably performed two or more times, with the alloy being dried between the steps. When the hydroxide treat ment is performed more than once, the total treatment time for all of the steps should be the amounts indicated above.

Subsequent to each treatment with alkali metal hydroxide, the alloy is subjected to a heating step. Such heating is accomplished in the presence of oxygen to convert the aluminum hydroxide to aluminum oxide. The temperature should also be suflicient for such conversion to take place. In the preferred embodiment, the catalyst is first dried at a temperature in the range of about 60 to 250 C. for a time period of at least about ten minutes.

During oxidation, the alloy is heated to a temperature in the range of about to 450 0., preferably a temperature toward the upper end of this range, for a time period of not less than 20 minutes, and preferably for at least about 30 minutes.

The amount of oxygen present in the gas passed over the catalyst during the oxidation step depends upon a number of factors, including the temperature. That is, the higher the temperature, the lower the concentration of oxygen required. Also, when it is desired to prepare a surface high in alumina, it is preferred that the stream passed over the alloy be relatively high in oxygen content. On the other hand, when a surface relatively low in alumina and high in the oxides of other metals contained in the catalyst is desired, such as cobalt, molybdenum, manganese, and/ or chromium oxides, the amount of oxygen in the stream should be relatively low. As a general matter, the oxygen may be varied anywhere from about 2 to 75% by volume. The remainder of the gas stream is preferably an inert gas, such as nitrogen, helium, or the like.

Subsequent to the oxidation step, where maximum surface area and porosity of the alumina are desired, it is preferred to calcine the alloy in order to convert the alumina to the gamma crystalline form. Such calcining is accomplished by heating the catalyst to a temperature of about 300 to 750 C. and preferably about 500 C. to 750 C. for a period of at least about 30 minutes. Such conversion of the alumina to the gamma form is particularly desirable where it is contemplated that the catalyst will be employed alone, or in combination with a noble metal, and where the alumina itself will have a significant catalytic effect. On the other hand, where the catalyst is to be coated with other catalytic metals, and is to be used only as a support, calcining of the catalyst is less important.

Virtually any catalytic metal may be applied to the catalyst of the present invention. Thus, the catalyst of the present invention may be used as a high surface-area support for virtually any of the well known catalytic metals. Included among such catalysts are the metals of Group VIII of the Periodic Table, the rare earths, and other metals including silver, titanium, manganese, copper, chromium, cadmium, molybdenum, vanadium, tungsten, rhenium, thorium, and actinium. Various combinations of these catalysts are also advantageously employed in a wide variety of catalyst applications. A particularly important application of catalysts of the present invention is in air pollution control applications, wherein it is desired to oxidize or reduce gases in order to destroy pollutants.

As an example of various catalysts that may be prepared in accordance with the present invention, the catalyst without any additional catalytic metal deposited performs as an excellent oxidation catalyst, as a result of the presence of gamma alumina. However, its performance is improved by the presence of a Group VIII noble metal. An oxidizing catalyst having a preference for certain hydrocarbons may be prepared, for example, by depositing manganese and cobalt on the surface. A cracking catalyst may be prepared by depositing, for example, a combination of manganese, cobalt, and copper..,A catalyst having characteristics lying between those of a cracking catalyst and of an oxidation catalyst may be prepared, for exam ple, by depositing upon the surface a combination of molybdenum, titanium and chromium. An almost infinite variety of other catalysts may be prepared, as will be appreciated by those skilled in the art.

The deposition of catalytically active metals upon the catalysts of the present invention may be accomplished either by electroplating or by chemical deposition, i.e., by chemical reduction of salt solutions. Both of these methods are well known in the art.

When the catalytically active metal is deposited by electrodeposition, it will probably be deposited primarily in the crevices or valleys, since the alumina in the catalyst will have a relatively low conductivity. Thus, a catalyst which has exposed alumina may be prepared. On the other hand, when the metal is deposited by the reduction of salt solution, it is deposited evenly over the entire surface area. Depending upon the amount deposited, the alumina may or may not be exposed. Exposed alumina is particularly desirable in the case of oxidation catalysts, but is relatively unimportant in many other catalyst applications.

It is also well known in the art that catalytic metals must generally be activated by conversion to the oxide form, and by contacting them with a stream of hydrocarbons subsequent to deposition. Such activation may also be performed in accordance with the present invention by procedures which are well known in the art.

In addition to the aforementioned catalytically active metals that may be applied to the surface of the catalysts prepared in accordance with the present invention, an additional layer of alumina may also be applied. Such an additional layer of alumina is particularly desirable, for example, where an alumina catalyst is desired, or where a catalyst high in alumina content and containing an additional catalytic metal, such as a Group VIII noble metal, is desired. In order to apply additional alumina to the catalyst, an additional layer of aluminum is first electroplated onto the catalyst by well known techniques. Such electroplating is conducted subsequent to the completion of the catalyst from the base alloy, which completion would ordinarily include the calcining step where active alumina is desired. The catalyst containing this additional layer of metallic aluminum is then contacted with an alkali metal hydroxide, reheated, reoxidized, and recalcined as was the original catalyst. The repeating of these steps converts the additional aluminum deposited to gamma alumina, thus producing a catalyst having a very high surface area, and having a surface which is almost entirely gamma alumina.

The following examples are intended to illustrate the present invention, and should not be construed as limitative, the scope of the invention being determined by the appended claims.

EXAMPLE I The alloy employed in this example contained 70.85% of iron, 22.0% chromium, 4.5% aluminum, 1.0% manganese, 1.0% silicon, 0.5% cobalt, and 0.15% carbon, all of the foregoing percentages being by weight. The alloy was in the form of a ribbon having a width of about 4 millimeters, and a thickness of about 0.35 to about 0.45 millimeter. This ribbon was crimped, cut into pieces 5 centimeters in length, and placed between a 3-mesh and a l5-rnesh screen. The screen material had a low aluminum content, as a high mechanical strength was desired.

The catalyst mat thus prepared was immersed in nitric acid having a concentration of about by weight. The temperature was maintained in the range of about 50 to 75 C. The acid was continuously agitated with a recycle pump, and was filtered as it was recycled. The alloy was maintained in contact with the acid for 12 minutes by being continuously dipped into the acid, removed, and reimmersed, in order to allow the dissolved metal, primarily iron, to run off. Fresh acid was introduced into the recycle stream in order to maintain the acid concentration in the range of 8 to 15% by weight. Spent acid was withdrawn to maintain a constant volume in the system.

Subsequent to the acid treatment, the alloy was rinsed, and was then immersed in an aqueous solution of sodium hydroxide having a concentration of 87% by weight. The temperature was maintained in the range of 45 to 65 C. and the treatment time was about 10 minutes. In order to avoid disturbing the delicate film of aluminum hydroxide, no agitation was employed during this alkali treatment. After treatment, excess liquid was permitted to drip on the alloy, and the mat was then placed in a stream of hot air at a temperature of 150 C. This temperature was gradually raised to 300 C. over a time period of about 10 minutes. The 300 C. temperature was main tained for 30 minutes, in order to completely oxidize the aluminum hydroxide to aluminum oxide. The immersion in the sodium hydroxide solution was then repeated two more times, and after each repetition the aforementioned drying and oxidation steps were repeated. After the final drying step, the oxygen in the air stream was enriched to a total oxygen content of 35-40% to insure complete oxidation. Subsequently, the temperature was raised to 500 C., and maintained at this level for 30 minutes, in order to convert the alumina to the gamma form.

The catalyst prepared as above was employed as an oxidation catalyst to oxidize an air stream contaminated with carbon monoxide to carbon. dioxide, and showed excellent catalytic activity.

EXAMPLE II A platinum catalyst was deposited upon the catalyst prepared in accordance with Example I according to the following procedure. First, 18.75 grams of chloroplatinic acid (H PtCl were dissolved in 300' grams of water and neutralized with sodium carbonate. This solution was then diluted to allow the application of 0.5 to 1.0 gram of PtCl equivalent to every square centimeter of surface area of the catalyst. A second solution was prepared by dissolving 15 grams of sodium carbonate and 10 grams of sodium formate in 300 ml. of water, and by diluting this solution to 2700 ml.

The chloroplatinic acid and sodium formate solutions were combined, and the catalyst prepared in accordance with Example I was inserted into the solution. The catalyst remained immersed in the solution for 20 minutes, with agitation. The deposition of platinum occurs in accordance with the following reaction:

The catalyst was removed from-the solution and heated in air at C. for 30 minutes, and then washed in cold water to remove residual sodium chloride. The catalyst was then heated in air at 350 C., the temperature being gradually increased to 500 C. over a time period of about 30 minutes. To activate the catalyst, about l2-15 grams of toluol per cubic meter of air were periodically added to the air stream. This procedure activates the catalyst by changing its crystalline structure, as is well known in the art.

Obviously, many modifications and variations of the invention as hereinbefore set forth will occur to those skilled in the art, and it is intended to cover in the appended claims all such modifications and variations as fall within the true spirit and scope of the invention.

I claim:

1. A method for preparing a metallic catalyst having a high surface area from an alloy comprising up to about 25% aluminum and a second metal, said second metal being soluble in oxidizing acids, said method comprising: contacting said alloy with an oxidizing acid for a time period and at a temperature and acid concentration sufiicient to dissolve a portion of said second metal; subsequently contacting said alloy with an alkali metal hydroxide under conditions sufiicient to convert aluminum at the surface of said alloy to aluminum hydroxide; and heating said alloy in the presence of oxygen for a time period and at a temperature sufiicient to convert said aluminum hydroxide to aluminum oxide.

2. The method as defined in claim 1 wherein said oxidizing acid is nitric acid.

3. The method as defined in claim 1 further comprising the step of calcining said alloy subsequent to said heating at a temperature and for a time period sufiicient to convert said aluminum oxide to the gamma form.

4. The method as defined in claim 3 wherein said second metal is selected from the group consisting of iron, nickel, molybdenum, titanium, vanadium, tungsten, and mixtures thereof.

5. The method as defined in claim 4 wherein said second metal is iron.

6. The method as defined in claim 5 wherein said alloy further comprises up to about 32% chromium by weight.

7. The method as defined in claim 5 further including the step of applying a catalytically active metal to said catalyst.

8. The method as defined in claim 7 wherein said catalytically active metal is a noble metal from Group VIII of the Periodic Table.

9. The method as defined in claim 8 wherein said noble metal is electroplated onto said catalyst.

10. The method as defined in claim 8 wherein said noble metal is deposited by the chemical reduction of a salt solution.

11. The method as defined in claim 3 further comprising steps of electroplating an additional layer of aluminum onto said alloy subsequent to said calcining; contacting said alloy and said additional layer of aluminum with an alkali metal hydroxide under conditions sufficient to convert said additional layer of aluminum to aluminum hydroxide; preheating said alloy and the presence of oxygen for a time period and at a temperature sufiicient to convert said aluminum hydroxide to aluminum oxide; and calcining said catalyst subsequent to said reheating at a temperature and for a time period sufficient to convert said aluminum oxide to the gamma form.

12. The method as defined in claim 1 further comprising the step of applying a catalytically active metal to said catalyst.

13. The method as defined in claim 12 wherein said catalytically active metal is electroplated onto said catalyst.

14. The method as defined in claim 12 wherein said catalytically active metal is deposited by the chemical reduction of a salt solution.

1 5. The method as defined in claim 1 wherein said second metal is selected from the group consisting of iron, nickel, molybdenum, titanium, vanadium, tungsten, and mixtures thereof.

16. The method as defined in claim 15 wherein said second metal is iron.

17. The method as defined in claim 16 wherein said alloy further comprises chromium.

18. The method as defined in claim 15 wherein said alloy further comprises a metal selected from the group consisting of chromium, cobalt, and manganese, and mixtures thereof.

19. A method for preparing a metallic catalyst having a high surface area from an alloy comprising about 4.5% to about 25% aluminum, by weight, and a second metal, said second metal being soluble in oxidizing acids, said method comprising: contacting said alloy with an oxidizing acid having a concentration of at least about 2.5% by weight for a time period of about 1 to 90 minutes and at a temperature of at least about 20 C., whereby to dissolve a portion of said second metal; subsequently contacting said alloy with an aqueous solution of an alkali metal hydroxide having a concentration of at least about 5% by weight for a time period of about 2 to minutes and at a temperature of about 20 to C., whereby to convert aluminum at the surface of said alloy to aluminum hydroxide; drying said catalyst at a temperature of about 60 to 250 C. for a time period of at least about 10 minutes; and heating said alloy in an atmosphere containing about 2% to 75% oxygen, by volume, for a time period of at least about 20 minutes and at a temperature of about 100 to 450 C.

20. The method as defined in claim 19 further comprising the step of calcining said alloy subsequent to said heating at a temperature of about 300 to 750 C. and for a time period of at least about 30 minutes.

21. The method as defined in claim 20 wherein said second metal is selected from the group consisting of iron, nickel, molybdenum, titanium, vanadium, tungsten, and mixtures thereof.

22. The method as defined in claim 21 wherein said second metal is iron.

23. The method as defined in claim 22 wherein said alloy further comprises up to about 32% chromium by weight.

24. The method as defined in claim 22 wherein said oxidizing acid is nitric and said alkali metal hydroxide is sodium hydroxide.

25. The method as defined in claim 22 further comprising the step of applying a catalytically active metal to said catalyst.

26. The method as defined in claim 25 wherein said catalytically active metal is a noble metal from Group VIII of the Periodic Table.

27. The method as defined in claim 26 wherein said noble metal is electrodeposited on said catalyst.

28. The method as defined in claim 26 wherein said noble metal is deposited by the chemical reduction of a salt solution.

29. The method as defined in claim 24 further comprising the steps of electroplating an additional layer of aluminum onto said alloy subsequent to said calcining; contacting said alloy and said additional layer of alumi num with an aqueous solution of alkali metal hydroxide having a concentration of at least 5% by weight for a time period of about 2 to 90 minutes and at a temperature of at least about 20 0., whereby to convert said additional layer of aluminum to aluminum hydroxide; redrying said catalyst at a temperature of about 60 to 250 C. for a time period of at least about 10 minutes; and reheating said alloy in an atmosphere containing about 2% to 75 oxygen, by volume, for a time period of at least about 20 minutes and a temperature of about 100 to 450 C.

30. The method as defined in claim 19 further comprising the step of applying a catalyticaly active metal to said catalyst.

31. The method as defined in claim 30 wherein said catalytically active metal is electroplated onto said catalyst.

32. The method as defined in claim 30 wherein said catalytically active metal is deposited by the chemical reduction of a salt solution.

33. The method as defined in claim 19 wherein said second metal is selected from the group consisting of iron, nickel, molybdenum, titanium, vanadium, tungsten, and mixtures thereof.

34. The method as defined in claim 33 wherein said second metal is iron, and wherein said iron is present in the range of about 6% to 85% by weight.

35. The method as defined in claim 34 wherein said alloy further comprises up to about 32% chromium.

36. The method as defined in claim 32 wherein said alloy further comprises a metal selected from the group consisting of chromium, cobalt, manganese, and mixtures thereof.

37. A method for preparing a catalyst having a high surface area from an alloy comprising about 4.5% to about 25% aluminum, by weight, and about 6% to about 85% iron by weight, said method comprising: contacting said alloy with nitric acid having a concentration of at least about 2.5% by weight for a time period of about 1 to 90 minutes and at a temperature of about 20 to 100 C., whereby to dissolve a portion of said iron; subsequently contacting said alloy with an aqueous solution of sodium hydroxide having a concentration of at least about 5% by weight for a time period of about 2 to 90 minutes at a temperature of about 20 to 100 C., whereby to convert the aluminum at the surface of said alloy to aluminum hydroxide; drying said catalyst at a temperature of about 60 to 250 C. for a time period of at least about minutes; heating said alloy in an atmosphere containing about 2% to about 75% oxygen, by volume, for a time period of at least about 20 minutes and at a temperature of about 100 to 450 C.; and calcining said catalyst at a temperature of about 300 to 750 C. for a time period of at least about 30 minutes.

38. The method as defined in claim 37 wherein said alloy further comprises up to about 32% chromium, by weight.

39. The method as defined in claim 38 further comprising the step of applying a catalytically active metal to said catalyst, said catalytically active metal being selected from the group consisting of metal from Group VIII of the Periodic Table, rare earths, silver, titanium, manganese, copper, chromium, cadmium, molybdenum, vanadium, tungsten, rhenium, thorium, actinium, and mixtures thereof.

40. The method as defined in claim 39 wherein said catalytically active metal is a noble metal from Group VIII of the Periodic Table.

41. The method as defined in claim 40 wherein said noble metal is electroplated onto said catalyst.

42. The method as defined in claim 40 wherein said noble metal is applied by the chemical reduction of a salt solution.

43. The methods as defined in claim 38 further comprising the steps of electroplating an additional layer of aluminum onto said alloy subsequent to said calcining; contacting said alloy and said additional layer of aluminum with an aqueous solution of sodium hydroxide having a concentration of at least about 5% by weight for a time period of about 2 to 90 minutes and a temperature of about 20 to 100 0, whereby to convert the aluminum at the surface of said alloy to aluminum hydroxide; redrying said catalyst at a temperature of about to 250 C. for a time period of at least about 10 minutes, and reheating said alloy in an atmosphere containing about 2% to oxygen, by volume, for a time period of at least 20 minutes and at a temperature of about -450 C.

44. A metallic catalyst prepared in accordance with the method of claim 19.

45. A metallic catalyst prepared in accordance with the method of claim 23.

46. A metallic catalyst prepared in accordance with the method of claim 25.

47. A metallic catalyst prepared in accordance with the method of claim 29.

48. A metallic catalyst prepared in accordance with the method of claim 37.

49. A metallic catalyst prepared in accordance with the method of claim 39.

50. A metallic catalyst prepared in accordance with the method of claim 43.

References Cited UNITED STATES PATENTS 3,488,226 1/1970 Baker et a1. 252-466 .1 3,448,060 6/1969 Mason 252-466 J 3,428,490 2/1969 Bravo ct a1 136-120 FC 3,150,011 9/1964 Winsel et a1 136-86 D 3,341,936 9/1967 Sanstede et a1. 13686 D 3,377,265 4/1968 Caesar 136120 FC DANIEL E. WYMAN, Primary Examiner P. E. KONOPKA, Assistant Examiner US. Cl. X.R.

208-112, 123, 124; 252-463, 464, 465, 466 B, 466 PT, 477 Q; 423-247 

